Light emitting diodes have been recently used in large volume in mobile handsets, digital cameras, personal digital assistants, traffic lights, automobiles, and so forth. Higher brightness is required before light emitting diodes can be used for other applications such as, for example, general illumination, as the brightness of the emitted light is not sufficient for light emitting diodes to be used in such applications.
There are a few reasons why the brightness and output power of light emitting diodes is limited. One of the major reasons is the limited light extraction efficiency. Due to total reflection at the interface between light emitting diode surfaces (semiconductor), or plastic encapsulation of a light emitting diode, the external efficiency of most light emitting diodes is limited to a few percent of input electrical power, while the internal efficiency is often as high as over 90%. The internal quantum efficiency characterises how many photons are generated for each electron passing through the light emitting diode. The extraction efficiency is the percentage of the generated light that escapes from the semiconductor light emitting diode.
Various methodologies have been developed to improve the light extraction efficiency, including:                (a) surface texturing (I. Schnitzer and E. Yablonovitch, C. Caneau, T. J. Gmitter, and A Schere, Applied Physics Letters, Volume 63, page 2174, October 1993);        (b) replacing the absorbing substrate with a non-absorbing substrate (F. A Kish, F. M. Steranka, D. C. DeFevere, D. A. Vanderwater, K. G. Park, C. P. Kuo, T. D. Osentowski, M. J. Peanasky, J. G. Yu, R. M. Fletcher, D. A. Steigerwald, M. G. Craford, and V. M. Robbins, Applied Physics Letters, volume 64, page 2839, 1994);        (c) providing a mirror on or within the substrate (R. H. Homg, D. S. Wuu, S. C. Wei, C. Y. Tseng, M. F. Huang, K. H. Chang, P. H. Liu, K. C. Lin, Applied Physics Letters, Volume 75, page 3054, November 1999) by wafer bonding or eutectic bonding;        (d) changing the chip geometry; and        (e) growing a semiconductor distributed Bragg reflector mirror (H. Sugawara, H. Itaya, G. Hatakoshi, Journal of Applied Physics, Volume 74, page 3189, 1993);and so forth.        
The reflective layer is normally on the rear surface—the surface to which the substrate is attached. This is because the ohmic contacts to the semiconductor are normally made on the front surface (especially for GaN light emitting diodes on a sapphire substrate), and light is emitted from the front surface and the edges of the substrate. To make this reflective layer requires a number of process steps. This is inefficient. Often the reflective layer reacts with the semiconductor layer and the solder materials, and adhesion of the reflective mirror layer to the underlying semiconductor is also a problem.
For GaN-based and AlGaInP light emitting diodes, the substrates are not good thermal conductive materials, and it is desirable to remove the substrate. In addition, the removal of the substrate simplifies the device fabrication with reflection mirrors, since the reflection mirror is on the front surface of the semiconductor, not on the back surface of the substrate, and acts also as the ohmic contact to the semiconductor front surface.
GaN blue laser performance is also improved by the removal of the substrate (W. S. Wong, M. A, Kneissl, U.S. Pat. No. 6,627,921 B2). Bonding the semiconductor epitaxial layers, either before or after the removal of the substrate from the semiconductor expitaxial layers, to a mechanical support (new substrate) by using eutectic bonding, wafer bonding, or by using organic bonding material, complicates the process and may limit the yield and throughput of the production. Often the bonding must be done at elevated temperatures. This may cause additional problems. For GaN light emitting diodes it is difficult to remove the sapphire substrate for the whole wafer and keep the epitaxial layer intact, making it difficult for large volume manufacturing by using bonding methods. This is because the bonding methods do not provide intimate and homogeneous bonding between the epitaxial layers and the new mechanical substrate, and the warping of the epitaxial layers on the new substrate causes cracks or stress in the epitaxial layers.